1. Field of the Invention
The present invention relates to a method and system for reducing noise in a jet engine. More particularly, the present invention relates to canceling the acoustic waves produced by a jet engine by stimulating a plasma within the engine to propagate an acoustic interference wave.
2. Prior Art
Jet aircraft are notorious for their excessive noise level. Individuals who reside near airports are continually subjected to high pitch, high volume noise from passing aircraft. Passengers on board the aircraft must also endure the noise for extended periods of time. This noise pollution is uncomfortable and even dangerous to the health of such persons. Accordingly, governmental regulations are in effect which require aircraft to adhere to certain maximum noise levels.
There are essentially three sources of acoustic noise in a gas turbine engine. The primary noise source is the engine exhaust. High velocity exhaust gases exiting the engine produce noise, as well as propagate mechanical noise from the engine. An additional noise source occurs at the air intake because of high speed rotation of the fan and compressor blades. Finally, mechanical engine noise is transmitted through the nacelle structures of the aircraft.
The most common methods for suppressing engine noise involve passive techniques, such as mechanically blocking the noise source with a muffler. Other passive techniques include mechanically blocking noise waves and/or converting the acoustic energy into different energy forms such as heat energy or acoustic waves of different frequencies. These techniques are only marginally effective because they allow the air and exhaust, which generate the noise, to flow freely from the source and then remedially attempt to cover or dissipate the acoustic energy. Similarly, exhaust systems have been developed that create a low velocity exhaust flowstream to surround, and thus block noise from, a high velocity exhaust flow stream. These techniques have had only limited success.
Other methods have focused on more active techniques, such as mixing inlet air with the exhaust gas to reduce the velocity of the exhaust gases. Such bypass systems involve varying the bypass ratio and depend on complex mixers for mixing the bypass air and exhaust gases. One problem with this technique is that it reduces the efficiency of the engine by bleeding intake air from the inlet.
Another active technique involves introducing a fluid, such as water, into the exhaust gases. The fluid is heated by the exhaust gas and expands rapidly. The rapid expansion produces sound and vibrations which are not in phase with the noise and vibrations produced by the engine. This method, however, requires the aircraft to carry a large volume of fluid, increasing the weight of the aircraft.
In theory, near complete suppression of engine noise should be possible with an active technique which cancels the acoustic waves. Such a technique would operate on the principle of wave interference or destructive interference. Methods that attempt to use destructive interference typically use a microphone, a control unit, and a speaker. The microphone senses the acoustic wave to be eliminated and generates an electrical signal that is sent to the controller. The controller inverts the signal and sends it to the speaker, which produces an acoustic wave that is usually 180.degree. out of phase with the original sound wave. The two waves interfere with and cancel one another.
Such techniques have been developed for integration on the exhaust of reciprocating engines and are described in U.S. Pat. Nos. 5,466,899 and 5,414,230. Unfortunately, implementing theoretical interference is usually frustrated by practical realities which limit formation of any ideal,phase inverted acoustic wave. Furthermore, these methods fail to cancel the first half cycle of the wave or the last half cycle. Even if inverse phase alignment were feasible, the two waves must still have symmetrical pressure variations for total cancellation.
A specific example of such a prior art is found in U.S. Pat. No. 3,936,606, issued on Feb. 3, 1976 to Wanke. This patent illustrates a noise suppression system for a gas turbine engine that produces an anti-wave, or phase inverted wave, which is in-phase and of mirror symmetry with respect to the acoustic wave. A microphone detects the acoustic wave. A controller creates a time delayed and phase reversed mirror symmetry signal to generate a phase inverted wave through a speaker. Wanke uses large ducts to channel the acoustic wave and anti-wave in order to create plane waves that can be easily combined to cancel each other. Practical application of this method is limited, however, because of the requirement that waves be directed through a wave guide to convert all the wave energy into plane waves.
Therefore, it would be advantageous to develop a method and apparatus for reducing noise capable of implementation on an aircraft and capable of creating an interference wave propagating in three dimensional space.